1. Technical Field
The present invention relates to security sensors of the type known as dual-technology sensors. Such sensors include both a passive infra-red intruder detector and a microwave intruder detector.
2. Prior Art
A sensor of this type is described in EP-A-No. 0 147 925 in the name of C & K Systems, Inc. The purpose of such sensors is to reduce the possibility of false alarms. The outputs from each detector are processed independently. However, an alarm signal is only generated if both detectors have been activated. Therefore this type of sensor can be defeated if one of the detectors is masked. In existing dual-technology sensors, the two independent detectors are mounted one above the other or side-by-side so that the overall dimensions of the sensor are considerably greater than those of either a passive infra-red sensor or a microwave sensor.